x86/resctrl: Remove rdt_cdp_peer_get()

Now that the configuration can be read from either resource, as they share
the ctrlval array, rdt_cdp_peer_get() is not needed to to map the resource
and search for the corresponding domain.

Replace it with a helper to return the 'other' CODE/DATA type, and use
the existing get-config helper.

Signed-off-by: James Morse <james.morse@arm.com>